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How to Read Transformer Curves

Transformers are electric devices that consist of two electromagnetically coupled inductors. Inductors are coils of wire that produce magnetic fields when current flows through them, but they also produce current when magnetic fields pass through them. A transformer works by sending current through an input inductor, which creates a magnetic field. This magnetic field is picked up by the output inductor, which produces current. The properties of a transformer vary with the frequency of the current. They are often plotted on graphs using curves. Reading a transformer curve can tell you a great deal about the performance of the transformer.

Things You'll Need

  • Transformer datasheet (see Resources)
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Instructions

    • 1

      Determine whether the transformer curve is plotted on a logarithmic or linear scale graph. This can be determined by reading the numbers that make up the vertical axis of the graph. If the numbers are evenly spaced (e.g. 1,2,3), then it is a linear graph. If the numbers are not evenly spaced (e.g. 1, 10, 100), then it is a logarithmic graph.

    • 2

      Read the horizontal axis, which displays the frequency of the alternating current that flows through the transformer. This axis shows the range of acceptable frequencies that the transformer was designed to handle. For example, a transformer might display frequencies that range from 1 to 200 Hz (cycles per second).

    • 3

      Determine what data the curve represents. The datasheet graph will have a heading, such as “Insertion Loss,” which is a measure of power lost between the input and output inductors.

    • 4

      Select one frequency on the horizontal axis. For example, if you are using the transformer for electrical home appliances, you would choose 60 Hz, since that is the frequency most appliances operate at.

    • 5

      Follow the vertical line up from the frequency you selected to the curve. Once you have found the intersection between the vertical line and the curve, follow the horizontal line over to the horizontal axis. This tells you the “Insertion Loss” at this frequency. If the intersection occurs in-between two values on the horizontal axis, you must estimate the actual value.

    • 6

      Estimate the actual value of the quantity if it occurs between two values on the horizontal axis. On a linear scale, these values are evenly spaced. But on a logarithmic scale they are spaced with a variable distance between them. This can make reading values especially hard, and you might need a straight-edge or ruler to see the exact location on the graph.

    • 7

      Look at some of the other graphed curves. Common curves on transformer datasheets are “Voltage Ratio” and “Return Loss.” Voltage ratio is the ratio of input voltage to output voltage, and is dependent on the number of coils in the inductor and the frequency. Return loss is a measure of signal power loss due to a mismatch in source and load impedance.

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